The present invention relates generally to preimpregnated fiber material, or prepregs, for making fiber reinforced composite structures, and more specifically to a slurry of powdered thermoplastic resin and appropriate additives for making thermoplastic resin prepregs having tack, drape and easy handleability.
Advanced, or high-performance, composites are high modulus (stiffness), high strength composite materials used in applications requiring high strength to weight ratios. They are typically made by embedding very high stiffness fibers, such as glass, carbon or silicon carbide, into a polymer matrix. The polymer matrix may be made either from thermosetting resins, such as an epoxy or polyester resin, or from thermoplastic resins, such as nylons, polycarbonates, acetals, polyethylenes, some polyesters, polysulfone and polyetheretherketone. Thermosetting resins take a permanent set when molded, and cannot thereafter be remolded. Thermoplastic resins become soft and pliable when heated and may be remolded without changing their physical properties. The embedded fibers provide stiffness and strength to the composite material, but are brittle and highly sensitive to cracks and flaws. The matrix material absorbs energy and hinders and spread of small cracks. The presence of a matrix allows a closer approach to using the theoretical maximum strength of the fibers in practical applications.
One method for making composite material is by impregnating an assembly of fibers with a flowable blend of resin powder and other materials, and then drying to produce a preimpregnated fiber material, commonly called a prepreg. Prepregs are fabricated into desired structural shapes by methods such as laying up and then curing under heat and pressure to make a structure component.
Unfortunately, conventional prepregs, particularly thermoplastic resin prepregs. (stickiness) or drape (tendency to fall onto and conform to the shape of a substrate). They are very difficult to handle in fabricating composite structures, particularly the complex shapes of aircraft and missile structures. They generally must be hot-pressed to fit and do not easily maintain either a desired position or shape during the lay up process.
The prior art includes at least one example of a drapeable prepreg called a commingled prepreg. Thermoplastic material is extruded to make a thermoplastic fiber that is woven with the reinforcing fibers to make a fabric. Under heat and roller pressure, the thermoplastic melts to impregnate the reinforcing fibers. Unfortunately, commingled prepregs are expensive and difficult to make.
Thus it is seen that there is a need for thermoplastic resin prepregs having tack, drape and easy handleability, properties now generally available only with thermosetting resin prepregs, and for an efficient method of making them.
It is, therefore, a principle object of the present invention to provide a slurry of thermoplastic polymer resin and appropriate additives that, when impregnated into continuous fibers, produces prepregs having tack, drape and easy handleability.
It is a feature of the present invention that its disclosed polymer resin slurry can be made into adhesive tapes and films for bonding together composite and other structures.
It is an advantage of the present invention that it is straightforward, efficient and easily adaptable to present methods for making prepregs and adhesive tapes and films.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.